By 2030, ~70 million people in the USA will be >65 years of age and ~10 million will be >85 years old. With aging of our population, there will be an increased incidence of age-related muscle wasting and weakness (sarcopenia), which is a significant predictor of chronic disease and mortality in the elderly. Previously, we found that sarcopenia affects the diaphragm muscle (DIAm) with atrophy of more fatigable type IIx and/or IIb muscle fibers, and a reduction in maximum specific force, making the DIAm considerably weaker in old age. Important to the proposed studies, we found that in older rats there are fewer large phrenic motor neurons (PMNs) that comprise more fatigable motor units. This raises the intriguing possibility that DIAm sarcopenia results from the loss of larger PMNs, leading to denervation of type IIx and/or IIb muscle fibers and DIAm weakness. In support, we found that DIAm sarcopenia is associated with impaired performance of higher force airway clearance behaviors (e.g., coughing, sneezing), which may underlie the increased risk of airway infections in older adults. Premise: The underlying cause of age-related PMN loss is unclear. Certainly with breathing, smaller PMNs are recruited more frequently, and thus their energy demands are higher. Accordingly, it is not surprising that in preliminary studies, we found that mitochondria in smaller PMNs are more fused and have a higher volume density. In diseases such as amyotrophic lateral sclerosis (ALS), mitochondrial fragmentation appears to precede the loss of motor neurons, which is associated with decreased Mfn2 and increased Drp1 expression. Indeed, experimental interventions that support mitochondrial fusion or inhibit fission appear to ameliorate motor neuron dysfunction and degeneration in ALS models. It is also important to note that brain-derived neurotropic factor (BDNF) signaling through its high affinity receptor (TrkB.Fl) promotes motor neuron survival, and mitochondrial fusion, suggesting there may be a link. In support, TrkB.Fl is co-localized to the mitochondrial outer membrane, and in preliminary studies we found that enhanced BDNF/TrkB signaling promotes mitochondrial fusion in NSC34 cultured motor neurons. Conceptual Framework: We hypothesize that the age-related loss of larger PMNs is related to mitochondrial fragmentation, which results from decreased Mfn2 and increased Drp1 expression, and is mitigated by enhanced BDNF/TrkB signaling. Specific Aim 1: To test the hypothesis that size-dependent differences in mitochondrial morphology in PMNs are exacerbated with aging and relate to differences in Mfn2 and Drp1 expression. Specific Aim 2: To test the hypothesis that the age-related loss of larger PMNs is related to mitochondrial fragmentation, which results from decreased Mfn2 and increased Drp1 expression. Specific Aim 3: To test the hypothesis that enhanced BDNF/TrkB.Fl signaling in PMNs mitigates age-related PMN loss, through an effect on Mfn2 and Drp1 expression and mitochondrial morphology.